The variable seems to be a leftover from importing the function from
busybox-1.23.2 and by cleaning up the code the compiler does not emit the
following warning anymore:

    src/base64.c:34:14: warning: variable ‘src_tail’ set but not used [-Wunused-but-set-variable]
      const char *src_tail;
                  ^~~~~~~~

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>